1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having at least two circuit blocks which are independently energized through different power supply systems and, more particularly to a protection circuit against an application of abnormally large electrostatic voltage to an external signal terminal.
2. Description of the Related Art
In a semiconductor integrated circuit having at least two circuit blocks each having MOS field effect transistors, a power supply is performed to energize the respective circuit blocks through different power supply systems in which different power supply lines are allocated to the respective circuit blocks and formed on the same integrated circuit chip. This structure is advantageous in suppressing noise on a power wiring that is generated by a change in current caused by change in number of circuits formed on the same chip. The separated power supply terminals ensure the independency of the respective current flow through each power supply wiring to prevent or reduce the voltage change on the power supply wiring based on a change in the current.
In the integrated circuit employing the above separated power supplying structure, an external signal terminal of one circuit block is provided with a protection circuit which is coupled to at least one of a high power voltage terminal and a low power voltage terminal that are connected to the same circuit block. The protection circuit discharges abnormally large electrostatic charges applied to the external signal terminal to the high power voltage terminal or the low voltage terminal through the corresponding power wiring when the voltage terminals associated with this circuit block are connected to power voltage sources, and to a stray capacitance of the power wiring when the associated power voltage terminals are not connected to the power voltage sources. However, the amount of the stray capacitance of the respective power wirings in the separated power supplying structure is remarkably small as compared with the power wirings in the conventional single power supplying structure employing the same power supply wiring for all the circuit blocks. Therefore, if abnormal electrostatic charges are applied to an external signal terminal of one circuit block when the power voltage terminals associated therewith are not connected to power sources, it is difficult to discharge the electrostatic charges to the stray capacitance of the associated power wiring sufficiently, resulting in destruction of a transistor, particularly a gate insulating film thereof in the above one circuit block.